The present invention relates to a drum and a stencil for use in a stencil printer and, more particularly, to an ink holding member and a stencil for a stencil printer and capable of effectively obviating the transfer of ink from the front of an underlying paper to the rear of an overlying paper.
A digital stencil printer generally includes a thermal head for perforating a thermosensitive stencil in accordance with data representative of a document image. The perforated stencil or master is wrapped around a rotatable drum. The drum is made up of a hollow cylindrical porous support, and an ink holding layer implemented as, e.g., a mesh screen of resin fibers or metal fibers. Ink feeding means is disposed in the drum and feeds ink to the drum. A press roller or similar pressing member presses a paper against the master wrapped around the drum. As a result, the ink is transferred from the drum to the paper via the pores of the drum and the perforations of the master, thereby printing the document image on the paper. The stencil is made up of a thermoplastic resin film (as thin as about 1 .mu.m to 2 .mu.m) and a porous substrate adhered to each other. The porous substrate is implemented by Japanese paper fibers or synthetic fibers or a mixture thereof.
It is a common practice with the above printer to use oil ink or water-in-oil type emulsion ink which is sparingly volatile. This kind of ink obviates defective printings ascribable to evaporation which occurs when the printer is operated after a long interval or after a printing operation. However, such an ink does not dry easily. This brings about a problem that a substantial period of time is necessary for the ink transferred to the paper to infiltrate into the paper and dry to such a degree that it does not run when, e.g., rubbed by finger.
In the stencil printer, papers carrying images thereon, i.e., printings are sequentially stacked on a tray. When the following paper is immediately stacked on the preceding paper existing on the tray, the ink is transferred from the front of the preceding paper to the rear of the following paper due to the short drying time. As a result, the rear of the following paper is smeared by the ink. This kind of smearing, i.e., rear smearing as referred to hereinafter often occurs when the ink transferred to the paper and forming an image is great in amount, particularly when the ink layer on the paper is thick, i.e., high.
Moreover, in the conventional stencil printer, the porous support and ink holding layer of the drum each has pores (bores) greater in size than perforations to be formed in the stencil by the head. Therefore, it is almost impossible to reduce the amount of ink transfer to the paper in order to obviate rear smearing.
In addition, the conventional porous substrate and mesh screen includes many portions which cause the incoming ink to simply flow out without being deflected. In such portions, the ink is transferred to the paper in a great amount and brings about rear smearing.
Japanese Patent Laid-Open Publication Nos. 1-204781 (corresponding to U.S. Pat. No. 4,911,069), 59-218889, and 1-267094, Japanese Patent Publication No. 63-59393, and Japanese Utility Model Publication No. 5-41026 are the prior art appearing to be relevant to the present invention.